1. Field of the Invention
The present invention relates to a photothermographic material and a method of forming an image using the photothermographic material. More particularly, the invention relates to a photothermographic material and an image forming method which provide excellent coating property and transportability during thermal development in a thermal developing apparatus.
2. Description of the Related Art
In recent years, it has been strongly desired in the field of films for medical imaging to reduce the amount of used processing liquid waste in consideration of environmental protection and space saving. For this reason, technology regarding photothermographic materials as films for medical imaging and for photographic applications, which are capable of efficient exposure with a laser image setter or a laser imager and capable of forming a clear black-toned image with high resolution and high sharpness is desired. Such photothermographic materials can eliminate use of liquid processing chemicals and can provide users with a thermal development system which is simpler and does not contaminate the environment.
Although similar requirements also exist in the field of general image forming materials, an image for medical imaging requires a particularly high image quality excellent in sharpness and granularity because a delicate image representation is necessitated. Also an image of blue-black tone is preferred in consideration of easy diagnosis. Currently various hard copy systems utilizing pigments or dyes, such as ink jet printers and electrophotographic systems, are available as general image forming systems, but they are not satisfactory as output systems for medical images.
On the other hand, thermal image forming systems utilizing organic silver salts are described, for example, in United States Patent U.S. Pat. Nos. 3,152,904 and 3,457,075, as well as in “Thermally Processed Silver Systems”, written by D. H. Klosterboer, appearing in “Imaging Processes and Materials”, Neblette, 8th edition, edited by J. Sturge, V. Warlworth, and A. Shepp, Chapter 9, pages 279 to 291, 1989. More specifically, a photothermographic material generally comprises an image forming layer in which a catalytically active amount of photocatalyst (for example, a silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt) and, if necessary, a toner for controlling the tone of a developed silver image are dispersed in a matrix of a binder. The photothermographic material, when heated at high temperature (for example, 80° C. or higher) after image exposure, forms a black-toned silver image by an oxidation/reduction reaction between the silver halide or the reducible silver salt (functioning as an oxidizer) and the reducing agent. The oxidation/reduction reaction is promoted by a catalytic effect of a latent image formed by exposure on silver halide. As a result the black silver image is formed in an exposed area (see, for example, U.S. Pat. No. 2,910,377 and Japanese Patent Application Publication (JP-B) No. 43-4924). Further, Fuji Medical Dry Imager FM-DP L is an example of a practical medical image forming system using a photothermographic material that has been marketed.
In production of a thermographic system using an organic silver salt, two methods are known. In one method, a solvent coating is adopted, and in the other method a coating liquid containing polymer fine particles as a main binder in an aqueous dispersion is applied and dried. In the latter method, since no necessity arises for a process of solvent recovery or the like, a production facility is simple and the method is advantageous for mass production.
In these methods, the coating property is a very important factor in the production process. A simultaneous multi-layer coating process can be applied for the aqueous dispersion system, and therefore efficient production can be attained. Improvement of the coating property is eagerly desired in order to allow more efficient production.
As described above, high quality is demanded for the formed silver image. In addition to the demand for high quality images, good physical characteristics of the materials are also eagerly demanded. This is because a material that achieves high quality images cannot be practically used if the material is scratched during conveying or cutting. Especially for medical diagnosis, meticulous care is required because the scratched material may lead to a mistake in diagnosis. In recent years, with a demand for rapid processing of a photothermographic material, the material is required to be conveyed by driving rollers at a high speed and to be conveyed while bending along a sharp curve needed for making a developing apparatus compact. Therefore, improvement in the physical characteristics of the material surface is increasingly desired.
In particular, a photothermographic material has a distinct difference from a photosensitive material used for a liquid development process in that a photothermographic material contains all chemicals necessary for development. Furthermore, after the development process, all used chemicals remain within the processed material. Accordingly, any additives to improve the physical characteristics of the material surface affect the other components included. Therefore, improvement of the surface properties by additives alone is very difficult, and intense consideration is required in regards to an influence of additives on all components included in the photothermographic materials.